Amyloidosis is a progressive, incurable metabolic disease of unknown cause characterized by abnormal deposits of protein in one or more organs or body systems. Amyloid proteins are manufactured, for example, by malfunctioning bone marrow. Amyloidosis, which occurs when accumulated amyloid deposits impair normal body function, can cause organ failure or death. It is a rare disease, occurring in about eight of every 1,000,000 people. It affects males and females equally and usually develops after the age of 40. At least 15 types of amyloidosis have been identified. Each one is associated with deposits of a different kind of protein.
The major forms of amyloidosis are primary systemic, secondary, and familial, or hereditary amyloidosis. There is also another form of amyloidosis associated with Alzheimer's disease. Primary systemic amyloidosis usually develops between the ages of 50 and 60. With about 2,000 new cases diagnosed annually, primary systemic amyloidosis is the most common form of this disease in the United States. Also known as light-chain-related amyloidosis, it may also occur in association with multiple myeloma (bone marrow cancer). Secondary amyloidosis is a result of chronic infection or inflammatory disease. It is often associated with Familial Mediterranean fever (a bacterial infection characterized by chills, weakness, headache, and recurring fever), Granulomatous ileitis (inflammation of the small intestine), Hodgkin's disease, Leprosy, Osteomyelitis and Rheumatoid arthritis.
Familial or hereditary amyloidosis is the only inherited form of the disease. It occurs in members of most ethnic groups, and each family has a distinctive pattern of symptoms and organ involvement. Hereditary amyloidosis is though to be autosomal dominant, which means that only one copy of the defective gene is necessary to cause the disease. A child of a parent with familial amyloidosis has a 50-50 chance of developing the disease.
Amyloidosis can involve any organ or system in the body. The heart, kidneys, gastrointestinal system, and nervous system are affected most often. Other common sites of amyloid accumulation include the brain, joints, liver, spleen, pancreas, respiratory system, and skin.
Alzheimer's disease (AD) is the most common form of dementia, a neurologic disease characterized by loss of mental ability severe enough to interfere with normal activities of daily living, lasting at least six months, and not present from birth. AD usually occurs in old age, and is marked by a decline in cognitive functions such as remembering, reasoning, and planning.
Between two and four million Americans have AD; that number is expected to grow to as many as 14 million by the middle of the 21st century as the population as a whole ages. While a small number of people in their 40s and 50s develop the disease, AD predominantly affects the elderly. AD affects about 3% of all people between ages 65 and 74, about 20% of those between 75 and 84, and about 50% of those over 85. Slightly more women than men are affected with AD, even when considering women tend to live longer, and so there is a higher proportion of women in the most affected age groups.
Several genes have been implicated in AD, including the gene for amyloid precursor protein, or APP, responsible for producing amyloid. Mutations in this gene are linked to some cases of the relatively uncommon early-onset forms of AD. Other cases of early-onset AD are caused by mutations in the presenilin genes, PS-1 and PS-2. A dementia similar to AD eventually affects nearly everyone with Downs syndrome, caused by an extra copy of chromosome 21. Other mutations on other chromosomes have been linked to other early-onset cases.
Potentially the most important genetic link was discovered in the early 1990s on chromosome 19. A gene on this chromosome, apoE, codes for a protein involved in transporting lipids into neurons.
Apolipoprotein E (ApoE) is a 34 kDa glycosylated protein. The main sites of ApoE production are the liver and brain. ApoE is a constituent of very low density lipoprotein (VLDL), a subclass of high density lipoproteins and chylomicrons. Cellular uptake of lipid complexes is mediated by binding of ApoE to the low density lipoprotein (LDL) receptor and other related receptors.
There are three major ApoE isoforms in humans, apoE2, apoE3 and apoE4 which are products of three alleles, ε2, ε3 and ε4. In the general population, the ε3 allele is the most common, accounting for 78% of all apoE alleles. The frequency of the ε4 allele is increased significantly in the population of late-onset sporadic and familial Alzheimer's disease (AD) patients.
ApoE contains a C-terminal domain (ApoE-CTD) and an N-terminal domain (ApoE-NTD) joined by a random-coil region. The C-terminal domain comprises a lipid binding site and the N-terminal domain binds to lipoprotein receptors. The CTD amino acid sequence is identical in all three isoforms of ApoE. The CTD and NTD may be separated by cleavage with thrombin.
Direct interactions between ApoE and Amyloid β (Aβ) have been demonstrated in vitro. ApoE is also present in AD plaques. It has been reported that the N-terminal domain of ApoE (ApoE-NTD) mediates binding of apoE to Aβ (Golabek et al., (2000) Biophysical Journal 79: 1008-1015). However, AD plaques containing ApoE have been shown to comprise full-length ApoE at the centre of the plaques and a C-terminal domain fragment of ApoE (ApoE-CTD) at the periphery of the plaques (Cho et al., (2001) J. Neuropathology and Expt. Neurology 60: 342-349). Aβ1-42 deposition in plaques has been shown to precede ApoE deposition whilst Aβ1-40 deposition follows ApoE depositor, in plaque maturation (Terai et al., (2001), Brain Research 900: 48-56).
The function of ApoE in the brain is not thought to be specific for AD. ApoE appears to play an important role in modifying recovery from acute brain injury. In particular, there is evidence from both clinical and animal studies to suggest that the presence of the ApoE4 isoform is associated with poor neurological recovery from a variety of acute brain injuries.